1. Field of the Invention
The present invention relates to a separator for a fuel cell and a manufacturing method thereof, and more particularly, relates to a metal separator of PEFC (Polymer Electrolyte Fuel Cell) and a manufacturing method thereof.
2. Description of Related Art
A PEFC apparatus includes individual fuel cells. Each fuel cell includes a membrane-electrode assembly (MEA) and a separator. The MEA includes an electrolyte membrane and a pair of electrodes disposed on opposite sides of the electrolyte membrane. The pair of electrodes includes an anode provided on one side of the membrane and constructed of a first catalyst layer and cathode provided on the other side of the membrane and constructed of a second catalyst layer. A first diffusion layer may be provided between the first catalyst layer and a first separator and a second diffusion layer may be provided between the second catalyst layer and a second separator. The first separator has a passage formed therein for supplying fuel gas (hydrogen) to the anode and the second separator has a passage formed therein for supplying oxidant gas (oxygen, usually, air) to the cathode. The separator constructs a passage of electrons between adjacent fuel cells.
At least one layer of the fuel cell constructs a module. A number of modules are piled, and electrical terminals, electrical insulators, and end plates are disposed at opposite ends of the pile of modules. After tightening the pile of modules in a fuel cell stacking direction, the end plates are coupled to a fastening member (for example, a tension plate) extending in the fuel cell stacking direction outside the pile of modules by bolts extending perpendicularly to the fuel cell stacking direction, thereby constructing a stack of fuel cells.
In the PEFC, at the anode, hydrogen is changed to positively charged hydrogen ions (i.e., protons) and electrons. The hydrogen ions move through the electrolyte membrane to the cathode where the hydrogen ions react with oxygen supplied and electrons (which are generated at an anode of the adjacent MEA and move to the cathode of the instant MEA through a separator, or which are generated at an anode of one of the axially outmost fuel cells and move to the cathode of the other of the axially outmost fuel cell through an outside electrical circuit) to form water as follows:At the anode: H2→2H++2e−At the cathode: 2H++2e−+(½)O2→H2O
Since the separator is required to have an electrical conductivity, the separator is made from metal, carbon, electrically conductive synthetic resin, or combination of metal and synthetic resin.
The carbon separator and the electrically conductive synthetic resin separator are chemically stable even when exposed to acid water and maintains the electrical conductivity for a long period of time. However, since it has to have a relatively large thickness for ensuring a strength at a bottom of the reactant gas passage when the passage is formed in the separator, a length of the fuel cell stack is necessarily large.
In contrast, with the metal separator, since the metal separator has a relatively large strength and the thickness at the bottom of the reactant gas passage of the separator may be thin, a length of the fuel cell stack is relatively small. However, since the metal separator suffers corrosion when exposed to acid water for a long period of time, there arises the problems of a decrease in the electrical conductivity due to the corrosion and a decrease of the electrical output of the fuel cell. Therefore, to use the metal separator, the corrosion resistance and the electrical conductivity of the metal separator should be assured for a long period of time.
Japanese Patent Application 2000-67881 discloses a separator for a fuel cell where an amorphous carbon layer, which is excellent in electrical conductivity and corrosion resistance, is formed directly on a surface of a base material of the separator by ion beam vapor deposition method.
However, with the conventional metal separator, there is a problem that since the carbon layer is formed directly on the metal separator base material, bonding of the carbon layer with the metal separator base material (SUS) is insufficient. When an acid water invades between the carbon layer and the base material at carbon layer peeled portions, metal of the base metal is melted in the form of ions into the water to attack the membrane electrolyte to shorten the life of the fuel cell.